Respiratory problems in horses may affect the upper or lower airways. The upper airways include the nostrils, the nasal passages, the guttural pouch and the larynx. The larynx is the junction between the nasal cavity and the trachea, and coordinates swallowing and protects the trachea and thus the lungs from the inhalation of food. The arytenoid cartilages, which are located in front of the trachea, are triangular-shaped, paired structures that are pulled open during inspiration to allow for an unobstructed airflow into the trachea. The larynx is innervated by the recurrent laryngeal nerves (RLN) which contain motor fibers that innervate both the abductor/opener and adductor/closer muscles of the arytenoid cartilages and their associated vocal folds. The larynx moves position when the horse swallows due to its attachments to the tongue and the basihyoid bone of the hyoid apparatus by the thyrohyoid membrane. The upper airway structures in a horse, and in particular the larynx, are susceptible to various disorders which affect the horse's health and its ability to perform normally since horses can only breathe through their nose.
Three common upper airway disorders affecting horses are left laryngeal hemiplegia, intermittent dorsal displacement of the soft palate, and epiglottic entrapment. In these disorders, normal airflow is compromised due to the collapse of tissue into the airway resulting in partial airway obstruction. For example, laryngeal hemiplegia is a distal axonopathy affecting the left RLN causing a unilateral disease. Damage to the left RLN compromises both the abductor and adductor functions by stopping vocal fold movement in a position just lateral to the midline. One surgical treatment for laryngeal hemiplegia is prosthetic laryngoplasty. In this treatment, the paralyzed left arytenoid cartilage is sutured in an open position to restore airflow. Conventional methods of treatment have been useful in some horses, but the treatments are clearly less than ideal since they have modest success rates, significant complications, and do not slow the progression of the disease. Thus, the disease often reaches a state where these methods do not help anymore.
Electrical stimulation has been successfully used for controlling weakened muscles/nerves, such as aged or degenerated nerves/muscles, for controlling re-innervating nerves, including synkinetically re-innervating nerves, and/or for providing electrical signals to nerves in order to compensate for hearing deficiencies (e.g., cochlear implant stimulation for providing hearing sensations to deaf people) or to overrule wrong elicited nerve signals.
Various stimulation systems and methods have been proposed to control the upper respiratory muscles, but none of them addresses the elevation of the larynx in order to provide open airways in horses. For example, Freed et al. describe a non-invasive method and apparatus that continuously stimulates the skin surface to assist patients in initiating a swallow (see, e.g., U.S. Pat. Nos. 5,725,564, 6,104,958, and 5,891,185). In addition, there are systems which cause glottis closure by means of appropriate electrical stimulation (see, e.g., Bidus et al., Laryngoscope, 110:1943-1949, 2000; Ludlow et al., Journal of Artificial Organs, 23:463-465, 1999; and Ludlow et al., Muscle and Nerve, 23:44-57, 2000). In U.S. Pat. Appl. No. 2007/0123950, Ludlow et al. disclose a method and system for synergistic production of muscle movements during speech, swallowing or voice production by moving the hyoid bone and/or parts of the upper airway and/or vocal tract by means of electrical stimulation of at least two different muscles. Ludlow et al. found that neuromuscular stimulation of only two of the muscles yields a large proportion of normal desired movement for the hyoid bone. Further, Ludlow et al. disclose that the muscles involved in swallowing remain at their normal, given locations within a human's body. In U.S. Pat. Appl. No. 2008/0208280, Lindenthaler discloses a method and system for treating an airway disorder in a horse that involves stimulating the airway tissue.